1. Field of the Invention
The present invention relates to an endoscope flexible tube and its manufacturing method for manufacturing a flexible tube constituting an insertion portion of an endoscope.
2. Description of the Related Art
Endoscopes for medical treatment for observing the insides of patient's body cavities are known. These endoscopes include an insertion portion to be inserted into patient's body cavities, and an operation portion provided at a distal end of the insertion portion. Inside the insertion portion, a treatment tool insertion tube, a light guide which transmits illumination light, a signal cable, and a bending wire which bends the insertion portion, etc. are provided from the distal side to the proximal side.
It is general that the flexible tube which is a main part which constitutes the insertion portion of the endoscope is composed of a spiral tube formed by winding a metallic strip spirally, a tubular net body which covers this helical tube, and a shell layer made of a urethane resin, etc. which is laminated on the surface of the tubular net body. In this case, it is preferable that the pliability at the distal side be high in order to make the insertion portion easily inserted into body cavities, and the pliability at the proximal side of the aforementioned flexible tube be low in order to facilitate operation. Thus, when the flexible tube is manufactured, as described in JP-UM-55-112505, it is suggested that two kinds of rigid and soft resins are used to form a shell layer having a two-layer structure of a soft resin layer and a rigid resin layer such that the ratio of the soft resin is high at the distal side, and the ratio of the rigid resin is soft at the proximal side. Further, in an endoscope flexible tube described in JP-A-2007-159775, a shell layer is constituted by only a soft resin layer at the distal side of the endoscope flexible tube, and the shell layer is constituted by only a rigid resin layer on the proximal side thereof.
Further, in order to efficiently the step of molding the shell layer of such a flexible tube, the configuration of an endoscope flexible tube and a manufacturing method which continuously performs molding of the shell layer while a plurality of flexible tubes are conveyed in a state where they are connected integrally is described in Japanese Patent No. 3586928.
In a case where the shell layer of the flexible tube as described in JP-UM-A-55-112505, and JP-A-2007-159775 is continuously molded as described in Japanese Paten No. 3586928, the molding is performed while the soft resin and the rigid resin are supplied to molding dies, respectively, by an extruder. Therefore, in order to constitute the outer shell with only either the soft resin or the rigid resin layer and set the thickness of the other thereof to zero, it is necessary to stop discharge of the resins by the extruder or to provide bypass passages which makes resins flow out of the molding dies.
However, it is very difficult to adjust the discharge amounts of the resins to the molding dies to zero, and since there are portions (welds) in which the resins stagnate inside the molding dies even if the discharge amounts are set to zero, it is difficult to set the thicknesses of the resins to zero. Moreover, in a case where the discharge amounts are increased gradually after the discharge amounts are set to zero, the resins which have stagnated inside the molding dies are extruded and molded. Therefore, the molding thicknesses of the resins in the circumferential direction of the flexible tube do not become uniform, but nonuniformity will be caused.